The invention relates to a system for a so-called “hill descent control” (HDC) for a motor vehicle, namely a system for controlling the speed of a motor vehicle driving downhill. In such a hill descent control, the invention relates, in this case, to the aspects “actuating the hill descent control” and “feedforward control of the downhill momentum as a disturbance variable”.
A hill descent control system is known from, amongst others, EP 0 856 446 B1. It is used to ensure the traction and driving stability of the motor vehicle when driving on steeply inclined roads, in particular off road. Said system is designed for a wheeled vehicle comprising a plurality of wheels, a plurality of brakes respectively intended for braking one of the wheels, an accelerator pedal, a brake pedal and a wheel lock sensor in order to detect locking of one of the wheels. A control unit has an activated state and a deactivated state. In its activated state each brake is actuated for braking the motor vehicle, when a detected motor vehicle speed is above a predetermined desired speed and no wheel locking is detected. One of the brakes is released when locking of the associated wheel is detected when the detected motor vehicle speed is above the desired speed. When entering the activated state, the control unit controls the brake such that the rate of acceleration of the motor vehicle reaches the desired speed without actuating the pedals if the motor vehicle speed is substantially less than the desired speed.
When entering the activated state, the control unit compares the motor vehicle speed with the desired speed and controls the braking means such that the motor vehicle speed approaches the desired speed. The rate of deceleration of the motor vehicle is controlled towards the desired speed when the motor vehicle speed is substantially greater than the desired speed. The rate of acceleration may be limited to a predetermined maximum value of approximately 0.2 to 0.3 g. The activated state may only be selected when the motor vehicle is in first gear or reverse gear. In the activated state, as a result of the actuation of a braking demand means of the motor vehicle by a driver, the control unit is overridden in order to increase the amount of braking above that provided by the control unit. Thus the motor vehicle is decelerated below the desired speed when no wheel locking is detected. The control unit may, in its activated state, release the brakes at least partially when the detected motor vehicle speed is below the desired speed. The control unit may be activated by a manually operable switch. The control unit actuates the brakes, if required, provided the detected motor vehicle speed is below the desired speed, in order to ensure that the rate of acceleration of the motor vehicle is less than a limit value.
In this case, the problem occurs, in particular, that soft and slippery ground conditions make driving with a motor vehicle considerably more difficult. As driving on steeply inclined roads generally takes place at low speed, a reduction and/or removal of the driving torque applied by the motor vehicle, even when the lowest gear is engaged, is insufficient on its own. Instead, additional braking moments have to be applied to the wheels.
Modern motor vehicles are generally equipped with an electrically controllable service brake system in order to enable brake functions which are independent of driver actuation, i.e. automatic brake functions, such as drive slip control (ASR) or driving dynamics control (ESP) to be carried out in addition to the anti-locking control function (ABS). To this end, the service brake system comprises a correspondingly constructed electrohydraulic control unit, an electronically controllable brake booster or is constructed as a so-called “brake-by-wire” (BBW) system. For the electronic control and/or regulation, an electronic control unit is provided which detects, via electronic sensors, variables related to operating conditions of the motor vehicle. Thus, for example, for the ABS control, the slip of the motor vehicle wheels is detected by means of wheel speed sensors, in order to control and/or to regulate the rotary behaviour of the motor vehicle wheels depending on the slip, such that locking is prevented.
WO 0114185A1 discloses a service brake system with hill descent control. A device for assisting the hill descent control detects operating conditions of the vehicle and, when driving on steeply inclined roads, adjusts additional braking moments irrespective of whether a brake pedal is actuated. The vehicle handling which results due to the adjustment of the manipulated variable, is continually detected using the vehicle speed and compared with a desired speed.
WO 9611826 discloses a service brake system with hill descent control which, when driving on steeply inclined roads, adjusts additional braking moments irrespective of whether a brake pedal is actuated, as soon as the vehicle speed exceeds a threshold value.